1. Field of the Invention
This invention relates to electrical cables and, in particular, to a high temperature electrical cable having an interstitial filler.
2. Description of Related Art
Electrical cables, such as wireline cables used in the oilfield industry, can often be subjected to temperatures exceeding 500° F. (260° C.), pressures exceeding 25,000 pounds per square inch (17.578 million kg/m2), and a wide range of destructive chemicals. Such extreme conditions have led to the design of electrical cables having conductors encased in insulating materials such as perfluoroalkoxy (PFA) polymers, polytetrafluoroethylene (PTFE), and polyimide polymers. Generally, such cables, for example a cable 100 shown in FIG. 1, include a central conductor 102 and a plurality of outer conductors 104 (only one indicated for clarity) arranged in a helical fashion surrounding the central conductor 102. In some such electrical cables, a tetrafluoroethylene (TFE) yarn 106 is disposed in interstices 108 between the outer conductors 104 to improve the roundness of the cable 100. Further, a braided tetrafluoroethylene sleeve 110 is disposed over the conductors 102, 104 and yarn fillers 106. Armor wire 112 is then wrapped over the braided tetrafluoroethylene sleeve 110 to give the cable 100 its desired breaking strength.
Such electrical cables have been found to have several shortcomings. As conventional high temperature cables lack fillers in interstices 114 between the central conductor 102 and the outer conductors 104, the insulation (e.g., the tetrafluoroethylene sleeve 110) surrounding the conductors 102, 104 can become deformed under high pressures, which can lead to electrical shorts and cable failures. High operating temperatures and forces coupled with high pressures can cause the conductor insulation to deform into the unfilled interstices 114, which can also lead to electrical shorts and cable failures. Manufacturing tetrafluoroethylene braiding can be time consuming and expensive. Tetrafluoroethylene yarns used in the braided sleeve 110 and as interstitial fillers can compress under pressure, creating additional voids into which conductor insulation may deform.
Further, as conventional cables are placed under axial tension and compression, the tetrafluoroethylene yarns in the braided sleeve 110 can act as a cutting device. This action can damage the insulation on conductors and lead to electrical shorts and failures. Applying armor wire 112 directly to tetrafluoroethylene sleeve 110 can result in improper seating of the armor wire 112, as the interstitial yarns 106 may not be held in place securely. Improperly seated armor wires 112 may cut into or otherwise damage components within the cable 100 itself.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.